1. Field of the Invention
The present invention relates to an image processing device for performing a format conversion, and more particularly relates to an image processing device that converts decoded video data in the 4:2:0 sampling format as defined by an MPEG standard into data in 4:2:2 sampling format and outputs the converted data.
2. Description of the Related Art
A moving picture's video data can be transmitted efficiently by compressing the data compliant with the MPEG-2 standard, for example. According to Main Profile @ Main Level, which is one of the most commonly used methods by the MPEG-2 standard, a luminance signal (Y) and color-difference signals (Cb, Cr) are sampled in a sampling format, in which these signals are sampled at a ratio of 4 to 2 to 0, thereby generating video data, which is then encoded with a frame structure compatible with non-interlaced scanning. Meanwhile, TV receivers and other video appliances adopt interlaced scanning and are normally designed to sample and process a luminance signal (Y) and color-difference signals (Cb, Cr) in a sampling format in which they are sampled at a ratio of 4 to 2 to 2.
That is why in processing video data generated by Main Profile @ Main Level, conventional image processing devices interpolate the color-difference signals decoded, thereby converting the video data in the 4:2:0 sampling format into data in the 4:2:2 sampling format and also outputting the decoded data with the scanning methods changed into the interlaced one.
A technique for converting video data in the 4:2:0 sampling format into data in the 4:2:2 sampling format is disclosed in Japanese Patent Application Laid-Open Publication No. 10-23460.
FIG. 7 illustrates a conventional image processing device for interpolating a color-difference signal. First of all, a color-difference signal c is received at a color-difference signal input terminal 202 and then supplied to a line memory 207, which stores data about the color-difference signal c for one horizontal scanning period and then will output a delayed color-difference signal dlyc in another horizontal scanning period. An interpolated data generating section 208 receives the color-difference signal c and the delayed color-difference signal dlyc, newly generates an interpolated color-difference signal by performing interpolation on the two color-difference signals, and then outputs it through a color-difference signal output terminal 204.
Next, it will be described with reference to FIG. 8 how the interpolated data generating section 208 generates the interpolated color-difference signal. Specifically, to convert the 4:2:0 sampling format compliant with the MPEG-2 into the desired 4:2:2 sampling format, interpolation calculations are carried out. For example, to obtain an interpolated color-difference signal Cb1, the following calculation:Cb1=(Ca1×5+Ca2×3)/8is carried out. And to obtain an interpolated color-difference signal Cb2, the following calculation:Cb2=(Ca1×1+Ca2×7)/8is carried out. In this manner, the interpolated data generating section 208 receives the two color-difference signals supplied and performs vertical interpolation processing (i.e., calculates a distance average by interpolation), thereby generating an interpolated color-difference signal.
On the other hand, a luminance signal is received at a luminance signal input terminal 201 and then delayed for one horizontal scanning period by a line memory 209 so that the luminance signal and the color-difference signal will be output at the same time. And then it is output as a delayed luminance signal dlyy1 through a luminance signal output terminal 203. In this manner, video data in the 4:2:0 format is converted into an interlaced signal in the 4:2:2 format and the converted signal is output.
In this interpolation processing, the interpolation using the color-difference signal is carried out on only the color-difference signal, not on the luminance signal. A processing technique that also performs interpolation on a luminance signal could be processing for converting an interlaced signal in the 4:2:2 sampling format into a progressive signal in a finer 8:4:4 sampling format. According to such conversion processing, interpolation is carried out on a luminance signal using the luminance signal, and interpolation is carried out on a color-difference signal using the color-difference signal independently of the luminance signal.